It is well known that an important part of window manufacturing is the insulating glass unit. The insulating glass unit generally comprises two pieces of glass and a spacer which is bonded between the glass to create a transparent, insulating air space which reduces the heat transfer between the outside and the inside of a home or other structure. The spacer is typically of a hollow rectangular metal which may contain moisture and volatile organic chemical adsorbents which function to desiccate the interior space of the sealed insulating glass unit. The perimeter of the unit is generally sealed with a sealant such as a hot applied pumpable butyl sealant, one-part or two-part urethane sealants, and silicone or polysulfide sealants.
In recent years, the assembly of insulating glass units has become automated. For example, spacers are filled with the moisture and/or volatile organic chemical adsorbents by automatic dispensing equipment. Linear corner keys are inserted into each spacer at specific intervals which facilitates forming of the linear spacer assembly into a rectangular shape. The application of hot sealants such as hot applied pumpable butyl sealants may be achieved by a continuous on-line application system in which the butyl sealant is heated and applied using a heated pumping system to three sides of the spacer. The spacer is then folded via the corner keys to form a rectangle which is sandwiched between the two glass sheets to form the sealed insulating unit. The assembled unit is then re-heated and compressed as it passes through a finishing oven to seal the unit against moisture. However, the use of corner keys to join non-continuous spacers has been known to make the insulating glass unit vulnerable to moisture intrusion.
An alternative spacer frame for an insulating glass unit is described in U.S. Pat. No. 5,177,916, which is marketed by PPG Industries, Inc. under the designation INTERCEPT.TM.. The unit utilizes a one-piece, U-shaped metal spacer between glass sheets. The spacer is notched to allow the spacer to be bent into a rectangular shape, which eliminates the need for corner keys. The spacer frame design provides improved moisture and gas impermeability and thermal efficiency. However, because of the design of the U-shaped channel, the moisture and chemical adsorbents must be incorporated in a mastic composition which is applied to the inner bottom surface of the U-channel. Further, the composition must be able to be dispensed using conventional pumping systems.
The composition must also contain sufficient moisture adsorbent to desiccate the interior air space of the sealed insulating glass unit effectively over the life of the unit. Further, the composition should not contribute to chemical "fogging", which is an unsightly accumulation of volatile organic chemicals that deposits on interior surfaces of the glass sheets. Such fogging can be caused by volatiles from the sealants, or volatiles from other structures in the interior of the glass assembly such as painted decorative grids.
One known mastic composition which is designed for use with the Intercept.TM. system is described in Meyer et al, U.S. Pat. No. 5,310,416. The composition contains polyisobutylene, amorphous polypropylene, tackifiers, and moisture and volatile organic chemical adsorbing materials. However, while such a composition is effective in keeping the air space dry and reducing chemical fogging, the composition is expensive to manufacture due to the high cost of polyisobutylene, which comprises up to 30% of the composition.
Accordingly, there is still a need in the art for an adsorbent containing composition which may be used in insulating glass units which is economical to produce, which effectively desiccates the space in the-unit, and which does not contribute to chemical fogging.